AT-WAVELENGTH CHARACTERIZATION OF AN EXTREME-ULTRAVIOLET CAMERA FROM LOW TO MID-SPATIAL FREQUENCIES WITH A COMPACT LASER-PLASMA SOURCE

The final alignment and characterization of an extreme ultraviolet (EU V) lithographic projection optics system must be performed at the oper ating wavelength in order to account for errors associated with the co mbined effect of multilayer coating and substrate fabrication error. T wo complementary metrology techniques have been developed which can op erate on a compact laser plasma source: EUV lateral shear interferomet ry (LSI) and aerial image monitoring (AIM), LSI quantifies residual lo w spatial frequency wave front errors associated with the ''figure'' o f each optical element. To characterize the effect of mid-spatial freq uency errors that lead to scatter in the image plane, an EUV aerial im age monitor was employed. EUV interferometry was compared to visible l ight interferometry measurements of the optical system and was found t o be in reasonable agreement for low spatial frequency errors. The con trast as measured by AIM and photoresist exposures indicates an additi onal reduction of contrast associated with scatter due to large mid-sp atial frequency errors. (C) 1997 American Vacuum Society.